The present invention relates generally to stationary roller shafts for gear assemblies for gas turbine engines and more particularly to an improved stationary roller shaft used in an air turbine starter for gas turbine engines.
An air turbine starter is a device used to start a gas turbine engine, such as an aircraft jet engine. The air turbine starter is mounted by the jet engine, much as a starter for an automobile is located by the automobile""s engine. In order to start the jet engine, the air turbine starter is activated. The owner of the present invention, Honeywell International Inc., has for years successfully designed, developed and manufactured air turbine starters.
An air turbine starter may include a high speed gear assembly that converts air flow into the rotational energy needed to start a gas turbine engine. This high speed gear assembly includes gears supported by rolling elements that rotate around a stationary roller shaft at speeds of about 26,000 rpm or more. Although prior air turbine starter designs are very safe and reliable, these high rotational speeds cause substantial Hertzian stresses and frictional shear loads on certain areas of the stationary roller shaft, which may result in wearing and fatigue at those areas. For years, stationary roller shafts of air turbine starters have been made from steel materials. However, because such steel materials generally have a hardness of 62 Rockwell C or less, they are susceptible to surface spalling and fatigue. Fabricating the stationary roller shafts from harder materials does not necessarily provide satisfactory results, as harder materials may also be more brittle and, thus, more susceptible to cracking. In addition, stationary roller shafts formed from steel materials with relatively high inclusion contents make the shafts susceptible to sub-surface rolling contact fatigue.
Accordingly, there is a need for an air turbine starter with a stationary roller shaft formed of material that is capable of substantially withstanding wearing and fatigue from rolling elements that rotate at high rotational velocities. The present invention satisfies this need and provides related advantages.
The present invention relates to a stationary roller shaft for an air turbine starter that is formed of a material that is capable of substantially withstanding wearing and fatigue from associated rolling elements that may rotate at high speeds.
In accordance with an exemplary embodiment of the present invention, and by way of example only, a stationary roller shaft of a gear assembly is provided. The gear assembly includes a plurality of rolling elements that contact a surface of the stationary roller shaft and are configured to rotate about the stationary roller shaft while the air turbine starter is in operation. The stationary roller shaft is formed of a material having a hardness of at least 66 Rockwell C and having a low inclusion content rating.
In accordance with another exemplary embodiment of the present invention, an air turbine starter is provided. The air turbine starter includes a housing defining an inlet, an outlet, and a flow path between the inlet and the outlet for conveying a flow of pressurized air therebetween. The air turbine starter also includes a turbine disposed in the flow path and which produces turbine shaft power. An output shaft transmits the shaft power externally of the air turbine starter. A gear train couples the turbine with the output shaft. The gear train comprises at least one stationary roller shaft that is formed of a material having hardness of at least 66 Rockwell C and having a low inclusion content rating.
In accordance with a further exemplary embodiment of the present invention, a gear assembly is provided. The gear assembly includes a stationary roller shaft and a plurality of rolling elements that are configured to rotate about the stationary roller shaft. The stationary roller shaft is formed of a material having a hardness of at least 66 Rockwell C and the material is produced by a process including vacuum induction melting, atomization, and hot isostatic compacting.
In accordance with yet another exemplary embodiment of the present invention, a method for fabricating a stationary roller shaft is provided. The method includes machining a stationary roller shaft from material produced by a process including vacuum induction melting, atomization, and hot isostatic compacting. The stationary roller shaft is tempered to a temperature in the range of about 500xc2x0 C. to about 600xc2x0 C. to achieve a hardness of at least 66 Rockwell C. The method further includes grinding the stationary roller shaft to a desired surface finish.
In accordance with yet a further exemplary embodiment of the present invention, a kit for installing a stationary roller shaft in a gear assembly is provided. The kit includes a stationary roller shaft and a plurality of rolling elements configured to rotate about the stationary roller shaft. The stationary roller shaft is formed of a material having a hardness of at least 66 Rockwell C and having a low inclusion content rating.
Other independent features and advantages of the preferred stationary roller shaft will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.